1. Field of the Invention
The present invention relates to disk drives. More particularly, the present invention relates to a headerless disk drive comprising repeatable runout (RRO) correction values recorded at a user data rate.
2. Description of the Prior Art
The above-referenced co-pending patent application Ser. No. 08/946,805 discloses a disk drive which stores correction values to attenuate repeatable runout (RRO), a disturbance in the servo control system due to eccentricities and other distortions in a track recorded on the disk. RRO is a predictable disturbance that is periodic with the rotation of the disk; therefore, it can be estimated and RRO correction values introduced into the servo loop as a correction signal in order to attenuate the RRO from the servo control effort.
It is known to calibrate and store the RRO correction values in embedded servo sectors during manufacturing of the disk drive. During user operations, the RRO correction values are retrieved from the embedded servo sectors and used to compensate for the RRO in the servo control system. However, the disk drive is designed to prevent writing to the embedded servo sectors during user operations in order to protect the integrity of the servo information. Thus, the prior art teaches that the RRO correction values are recorded in the servo sectors during manufacturing, after which the RRO correction values are not modified over the life of the disk drive.
In U.S. Pat. No. 5,825,578 a disk drive is disclosed wherein the RRO correction values may be stored in a header of a data sector. The header stores sector identification (ID) data which is used to verify the correct location of the head before reading or overwriting a data sector. In xe2x80x9cheaderlessxe2x80x9d disk drives (otherwise referred to as ID-less disk drives), sector headers are not used and instead the sector identification data is stored in the embedded servo sectors. The above-referenced U.S. Pat. No. 5,825,578 teaches that for headerless disk drives, the RRO correction values are stored in the embedded servo sectors.
FIG. 1A shows an example format of a disk 2 typically employed in a headerless, magnetic disk drive. The disk 2 comprises a plurality of concentric tracks 4 partitioned into a number of headerless data sectors 6 with embedded servo sectors 8 recorded at a regular interval around the disk 2. The data sectors 6 store the user data received from the host computer, and the embedded servo sectors 8 store servo information for use in positioning a head over a centerline of a selected track 4 while writing data to or reading data from the disk 2. The embedded servo sectors 8 also store sector identification data for identifying the data sectors 6 during read and write operations.
In order to increase the capacity of the disk drive, a more constant linear recording density is achieved by banding the tracks 4 into predefined zones and increasing the data rate from the inner diameter zones to the outer diameter zones. The data rate can be increased in the outer diameter tracks 4 due to the increase in circumferential recording area. The zoned recording technique is illustrated in FIG. 1A wherein the disk 2 comprises an inner zone 10 for storing fourteen data sectors 6 per track 4 and an outer zone 12 for storing twenty eight data sectors 6 per track 4. In practice, the disk 2 is actually partitioned into numerous zones with the data rate increasing incrementally from the inner to outer diameter zones.
The embedded servo sectors 8 are typically recorded at a lower data rate than the data sectors 6. The embedded servo sectors 8 are also typically not zoned but are instead recorded at the same data rate from the inner to outer diameter tracks 4. An example format of an embedded servo sector 8, as shown in FIG. 1B, includes a preamble field 14, a sync mark field 16, a servo information field 18, and servo bursts 20. An acquisition preamble (e.g., a 2T acquisition preamble) is typically recorded in the preamble field 14 which enables a read channel within the disk drive to acquire the appropriate timing and amplitude information from the read signal before reading the servo information. A sync mark (typically fault tolerant) is recorded in the sync mark field 16 and used to symbol synchronize the servo information stored in the servo information field 18. The servo information typically includes a servo track address which identifies the current track the head is over while the head is seeking to a selected track, as well as sector identification data used to identify the data sectors 6 of the tracks 4. The servo bursts 20 are groups of pulses recorded at precise intervals and offsets from a track""s centerline. The servo bursts 20 provide fine position information for the head with respect to the track centerline, and are used during read or write operations by the servo control system to maintain the head over the centerline of the selected track.
As described above, it is known to store the RRO correction values in the embedded servo sectors 8, either within the servo information field 18 or in a separate field (e.g., in a field following the servo bursts 20). However, because a disk drive is typically designed to prevent writing data in the servo sectors 8 in order to protect the integrity of the servo information, the RRO correction values stored in the servo sectors 8 cannot be modified during user operations. This technique requires that the RRO correction values calibrated during manufacturing be relatively accurate since there is no opportunity to fine tune the RRO correction values after manufacturing. This slows the manufacturing process since the accuracy of the RRO correction values is directly related to the number of revolutions used to compute the RRO correction values for each track. In addition, storing the RRO correction values in the write protected embedded servo sectors prevents adapting the RRO correction values to account for changes in the actual RRO that can occur over the life of the disk drive. Modifying the disk drive to allow data to be written to the embedded servo sectors during user operations is undesirable because of the aforementioned risk of corrupting the servo information which would render the user data recorded in the track unrecoverable. Further, designing a disk drive to allow data to be written to the servo sectors during user operations would be a significant departure from existing architectures, requiring substantial resources to develop and test.
There is, therefore, a need for a disk drive capable of storing RRO correction values in a cost effective manner without requiring significant modifications to existing disk drive architectures. Further, there is a need to update the RRO correction values in order to track variations in the actual RRO that can occur over the life of the disk drive.
The present invention may be regarded as a disk drive comprising a track recorded on a disk, wherein the track has repeatable runout (RRO). The track comprises a plurality of embedded servo sectors including servo information stored at a servo data rate, and a plurality of data sectors between the embedded servo sectors. The track is headerless in that the plurality of data sectors are uninterrupted by sector identification data. One of the plurality of data sectors includes a RRO correction value used to compensate for the RRO, the RRO correction value being stored at a user data rate different than the servo data rate. The disk drive further comprises a head, and a voice coil motor (VCM) for positioning the head over the track. A servo control system, responsive to the servo information and the RRO correction value, generates a VCM control signal that is applied to the VCM for positioning the head over the track.
The present invention may also be regarded as a method of compensating for repeatable runout (RRO) in a track recorded on a disk. The track comprises a plurality of embedded servo sectors including servo information stored at a servo data rate, and a plurality of data sectors between the embedded servo sectors. The track is headerless in that the plurality of data sectors are uninterrupted by sector identification data. One of the plurality of data sectors includes a RRO correction value used to compensate for the RRO, the RRO correction value being stored at a user data rate different than the servo data rate. A head is positioned over the track using a voice coil motor (VCM). The method further comprises the step of generating a VCM control signal in response to the servo information and RRO correction value, wherein the VCM control signal is applied to the VCM for positioning the head over the track.